This invention relates to a method for magnetic resonance spectroscopic imaging of a sample in which a total time of imaging can be reduced while allowing the acquisition of spectra from a large number of voxels within the sample and while maintaining an acceptable image quality.
Spectroscopic imaging (SI) is a technique that allows the acquisition of spectra from a large number of voxels in a single experiment and is thus very attractive for clinical use. The following documents provide an explanation of the general principles involved. The main impediment to routine clinical use is the long acquisition time.
A number of fast SI sequences have been developed, many of which, for example those shown in a paper by Pohmann, R., von Kienlin, M., Haase, A., in Journal of Magnetic. Resonance 129, 145-160, 1997, sacrifice spectral quality and spectral resolution for reduced acquisition times.
In a paper by Webb, A. G., Mareci, T. H., Briggs, R. W., in Journal of. Magnetic. Resonance. Ser. B., 103:274-277, 1994 is disclosed an arrangement proposing an acquisition-weighted algorithm, in which the number of averages per phase encode is varied, does not reduce spectral quality, but is only useful for applications where multiple averages per phase encode are used. Typical clinical use of proton spectroscopic imaging involves a large number of phase encodes with only one to two averages per phase encode, therefore this technique does not apply.
Another approach, disclosed in a paper by Kuhn, B., Dreher, W., Norris, D. G., Leibfritz, D., in Magn. Reson. Med., 35:457-464, 1996, to weight the data during acquisition is to vary the repetition time (TR) as a function of k-space position such that the low spatial frequency data, determining the overall contrast, are obtained with standard TRs and the higher spatial frequency data, defining the edges, are obtained with successively shorter TRs, resulting in a total acquisition time which is reduced compared to a standard acquisition technique. In this technique filtering of the k-space data is accomplished during acquisition, rather than during post-processing. The acquisition apodization function in this reduced TR technique thus depends on the T1 relaxation time of the metabolites. The primary limitation of the technique as it was proposed is the reduced spectral quality, as evidenced by poor spectral resolution, inherent to any technique that utilizes short acquisition times. The technique could be used with standard, long acquisition times but would result in minimal time-savings as compared to standard SI techniques.
A similar technique is disclosed in U.S. Pat. No. 5,202,632 of Kaufman et al issued April 1993.
It is one object of the present invention to provide a SI acquisition technique that minimizes loss of spectral quality while maintaining significant time-savings which may be as much as 40-50% as compared to standard SI.
According to the invention therefore there is provided a method of generating a spectroscopic image using magnetic resonance in a sample comprising:
locating a sample having a volume of interest in a magnetic field;
obtaining spectroscopic data from voxels within the volume of interest of the sample by subjecting the sample to repeated magnetic resonance experiments with incremented phase-encoding magnetic field gradients applied in one, two or three dimensions in which the sample is excited and phase-encoded during an excitation time, in which the sample emits signals in response to the excitation which are detected during a data acquisition time, and in which the sample magnetization relaxes towards equilibrium values during a subsequent delay time;
wherein each experiment is effected over a repetition time TR which is defined by the sum of the excitation time, the data acquisition time and any delay time prior to the next experiment;
wherein the TR is not kept constant for all experiments required to complete the acquisition of all phase-encoded signals, but is systematically reduced as a function of experiments encoding increasing spatial frequency within the volume of interest
wherein the data acquisition time is reduced for experiments encoding higher spatial frequencies relative to the data acquisition time for experiments encoding lower spatial frequencies of the volume of interest, thus allowing shorter TR periods for experiments encoding higher spatial frequencies while maintaining longer TR periods with longer data acquisition times for experiments encoding lower spatial frequencies
and wherein the data acquisition time for those experiments encoding low spatial frequencies is of sufficient length to obtain the desired spectral resolution, consistent with the decay of the signal due to transverse relaxation and magnetic field inhomogeneities.
The excitation time as set forth above may also include magnetization preparation such as spatial saturation and/or water suppression.